In recent years, stimulus-responding polymers are broadly applied to drug delivery systems (DDS), various types of separating agents, catheters, artificial muscles, chemovalves and the like, and their importance is rapidly increasing. For example, Japanese Patent Laid-Open No. 103653/1996 describes poly-N-isopropylacrylamide, N,N-diethylacrylamide and the like acrylamide derivatives and polymethylvinyl ether and the like vinyl ethers as stimulus-responding polymers which swell or contract in an aqueous solution by changing their higher-order structures through the stimulus of heat, pH, potential, light or the like.
However, though it is described that known polymers which swell or contract by responding to temperature changes have upper critical solution temperature (to be referred to as “UCST” hereinafter) or lower critical solution temperature (to be referred to as “LCST” hereinafter), all of them are polymers having LCST in reality. That is, they have a property to become insoluble in water at a certain temperature or more by reversibly causing aggregation between polymers but becomes soluble in water at less than that.
Since polymers having LCST become insoluble in water at a certain temperature or more by contraction of the polymers, there is a problem in that a demand for carrying out the contraction under a low temperature by a temperature falling operation is difficult to control, in applying them to separating agents and the like uses. For example, when used as separating agents for proteins and the like which are unstable to heat, the polymers having LCST aggregate by a temperature rising operation, so that it is necessary to take into consideration a danger of accompanying denaturation of protein by the operation.
On the other hand, in inter-polymer ion complex, polysoap and the like which use polyacrylic acid as a polymer that shows UCST in a aqueous solution, they do not show the UCST in some cases due to ion dissociation when they are used in a buffer solution for, e.g., separating agent and the like uses.